Molybdenum disulfide (MoS.sub.2) is a semiconducting layered compound which has attracted much attention as a catalyst, an electrode material and a lubricant. See, for example, E. Furimsky, Catal. Rev. Sci. Eng., 22, 371 (9180); D. L. Fleishaven, Thin Solid Films, 154, 309 (1987) and H. Tributsch, Faraday Discuss. Chem. Soc., 70, 190 (1981). Lubrication of moving parts in spacecraft under high vacuum conditions presents special problems, such as the need for a lubricant exhibiting low outgassing, zero creep, wide thermal operating parameters, and the lowest possible friction. Thin films of MoS.sub.2 are currently the solid lubricant of choice for high precision spaceborne applications such as satellite mechanism bearings, gears, gimbals and splines.. See, e.g., Design News, 43, 35 (Feb. 23, 1987).
The low shear strength of MoS.sub.2 can be explained by the material's crystal structure and bonding. MoS.sub.2 is comprised of hexagonally packed planes consisting of a layer of molybdenum (Mo) bounded on each side by a layer of sulfur (S). The strong bonding is within the "sandwich" or "basal" plane, and with weak van der Waals bonding between adjacent sandwich planes.
MoS.sub.2 films can be prepared which exhibit different crystal orientations. Studies have been reported by P. D. Fleischauer in ASLE Transactions, 27, 82 (1987) and in Tribiology Transactions, 31, 239 (1988) on the effects of crystal orientation on environmental stability and lubrication properties of MoS.sub.2. The friction mechanism of MoS.sub.2 is correlated with its crystal orientation and lubricant properties. It has been found that if the crystal structures are arranged with their basal planes parallel to the substrate surface, the films will have good stability (low reactivity) and longer endurance lives than films with randomly oriented crystal structures. It is proposed that films exposing basal planes are much more resistive to oxidation (which degrades lubricant properties), because the active oxygen or water can attack the bulk of the film (by reacting with Mo center, forming MoO.sub.3) only by entry through grain boundaries or breaks in the films. Furthermore, the friction and wear properties of films with basal orientations are superior, because the crystallites are initially oriented for maximum lateral slippage, whereas the randomly oriented films must suffer considerable crystallographic reorientation during the run-in or initial operation.
Although in many film deposition techniques such as rubbing or burnishing, sputtering, CVD, and laser pulsing, inorganic and organic binder-type sprays have been used to deposit MoS.sub.2 films on various supports, attaining films exhibiting a single orientation in concert with meeting industrial application specifications (thickness and adhesion) is still an unaccomplished goal of the solid lubricant industries. Potentially, sputtering is a better deposition technique; it avoids the use of organic binders which can outgas under the vacuum application conditions. For aerospace lubricant applications, M. Arita et al., Tribiology Transactions, 35, 374 (1992) have reported that sputtered MoS.sub.2 films showed the most resistance to oxidation by atomic oxygen. However, as examined by scanning electron microscopy, the crystal orientation of a typical sputter deposited MoS.sub.2 film is random and exhibits "columnar" surface structures. These columnar structures provide oxidation sites (grain boundaries) which lead to rapid degradation of the deposited MoS.sub.2 film.
Therefore, there is a continuing need for a method which can produce MoS.sub.2 thin films that satisfy all three of the following criteria: (1) purely basal oriented; (2) stoichiometric; and (3) sufficient thickness for industrial applications such as those discussed above.